The present disclosure relates to a method of exposing an extreme ultraviolet (EUV) photoresist, and an apparatus for implementing the same.
Extreme ultraviolet lithography is a lithography technology using an extreme ultraviolet (EUV) wavelength radiation for exposure. As used herein, EUV refers to a range of electromagnetic radiation having a wavelength from 10 nm to 50 nm. When an EUV photon is absorbed in a photoresist, at least one photoelectron, secondary electrons, and thermal electrons are generated by ionization. The photoelectrons are generated as a direct result of a photon-matter interaction between the EUV photon and the matter in the photoresist layer. The secondary electrons are caused by collision of the photoelectron with additional electrons as the photoelectron travels through the photoresist material. The thermal electrons are derived from the photoelectrons or the secondary electrons due to their energy loss or due to collisions that transfer energy less than about 2.5 eV.
EUV photoresist exposure (on a semiconductor wafer) is typically accomplished by generating photoelectrons within a photoresist layer. Upon generation, the photoelectrons do not have controlled directionality. As a result, feature edges defined by an EUV exposure are variable and dependent upon the path of the secondary electrons, their inelastic collisions, the resultant thermalization that ultimately drives the decomposition of the photoacid generator (PAG) within the photoresist, and the resist polymer/molecular distribution and homogeneity for reaction sites.
Thermalized electrons are estimated to have a mean free path of about 2 nm to 5 nm. Currently, the imaged feature edge roughness, as well as resolution of the minimum feature size, is insufficient for EUV lithography to be able to achieve the required performance for utilization in semiconductor manufacturing. To date, high volume semiconductor wafer exposure tooling has not had to address photoelectron directionality, as the resist exposure has been a photon induced reaction.